ELM329
Example Applications
The SAE J1962 (ISO 15031-3) standard dictates
that all OBD compliant vehicles must provide a
standard connector near the driver’s seat, the shape
and pinout of which is shown in Figure 8 below. The
circuitry described here can be used to connect to this
J1962 plug without modification to your vehicle.
which is likely enough for most applications. (The
versions without the ‘E’ are only rated to 120 Kbps.)
In all, the MAX3222 has been a great chip, but we
do caution you about one problem that we have
encountered. If pin 18 (the Shutdown pin) is held low
during powerup, then the chip will draw enough current
to drop the 5V supply to such a level that the ELM329
is not able to start up. Do not install any pull-down
resistors on pin 18 of the MAX3222! If you build the
circuit of Figure 9, you should not encounter any
problems.
1
9
8
16
The top left corner of Figure 9 shows the CAN
interface circuitry. We do not advise making your own
interface using discrete components – CAN buses
may have a lot of critical information on them, and you
can easily do more harm than good if you fail. It is
strongly recommended that you use a commercial
transceiver chip as shown. The Microchip MCP2551 is
used in our circuit, but most major manufacturers also
produce CAN transceiver ICs – look at the NXP
PCA82C251, the Texas Instruments SN65LBC031,
the infineon TLE7250G, and the Linear Technology
LT1796, to name only a few. Be sure to pay attention
to the voltage limits – depending on the application
you may have to tolerate 24V, not just 12V.
The voltage monitoring circuitry for the AT RV
command is shown connected to pin 2 of the ELM329.
The two resistors (R9 & R10) simply divide the battery
voltage to a safe level for the ELM329, and the
capacitor (C10) filters out noise. As shipped, the
ELM329 expects a resistor divider ratio as shown, and
sets nominal calibration constants assuming that. If
your application needs a different range of values,
simply choose the resistor values to maintain the input
within the specified 0-5V limit, and then perform an AT
CV to calibrate the ELM329 to your new divider ratio.
The maximum voltage that the ELM329 can show is
99.9V.
The only remaining components are the LEDs and
the crystal. The LEDs are standard ones, and may be
any colour that you require - we only offer suggestions
here. The crystal is a 4.000MHz microprocessor type,
while the 27pF loading capacitors shown are typical
only, (you may have to select other values depending
on what is specified for the crystal that you use). This
crystal frequency is critical to the circuit operation and
must not be altered. Do not substitute a resonator for
the crystal, as it will not have the accuracy required.
As always, you are not limited to the circuit of
Figure 9. It is only a starting point that you can build
Figure 8. The J1962 Vehicle Connector
The male J1962 connector required to mate with a
vehicle’s connector may be difficult to obtain in some
locations, and you might be tempted to improvise by
making your own connections to the back of your
vehicle’s connector. If doing so, we recommend that
you do nothing that would compromise the integrity of
your vehicle’s OBD network. The use of any connector
which could easily short pins (such as an RJ11 type
telephone connector) is definitely not recommended.
The circuit on page 69 (Figure 9) shows how the
ELM329 might typically be used. Circuit power is
obtained from the vehicle via OBD pins 16 and 5, and
after a protecting diode and some capacitive filtering,
is presented to a five volt regulator. (Note that a few
vehicles have been reported to not have a pin 5 – on
these you will need to use pin 4 instead of pin 5.) The
regulator powers several points in the circuit as well as
an LED (for visual confirmation that power is present).
We have shown an LP2950 for the regulator as that
type has very low quiescent current which is important
if you are going to use the low power feature of the
ELM329.
Note that there are some rather large capacitors
(C2 and C4) shown on the input and the output of the
regulator. Without these capacitors, you would likely
experience ‘LV RESET’s as the MAX3222E
transceiver is switched on and off. Testing has shown
that the values shown for C2 and C4 should be
adequate, but if you do see the occasional ‘LV
RESET’, you may want to increase them further.
The MAX3222E RS232 transceiver handles all of
the voltage translation and logic inversion needed to
provide standard RS232 communications between the
ELM329 and the controlling computer. The ‘E’ version
shown is capable of up to 250Kbps communications,
ELM329DSB
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